One major concern with respect to circuitized substrates adapted for working with electronic components (devices) such as semiconductor chips, modules, resistors etc. involves the effective dissipation of heat generated by such components. In many of today's substrate designs, such components are mounted on one or more external surfaces of the substrate and electrically coupled to selected internal circuitry, e.g., using plated through holes (PTHs). In some more recent products, components such as resistors, capacitors and even semiconductors, are incorporated within the substrate itself. Because today's technology often demands the use of many such components, including those required to operate at even greater capacities, increasing amounts of heat are being generated. To accommodate this, the use of dissipating heat sinks and the like structures have been used. These typically consist of projecting elements (e.g., fins) of a sound heat conductive material (e.g., aluminum or copper) which are thermally connected, either directly or indirectly, to the heat generating components or to a supporting member for same. These projections offer a relatively large surface area exposed to the product's ambient, across which cooling air may be directed to enhance heat removal. Examples of such heat sinks are described in U.S. Pat. Nos. 5,875,097, 6,181,556 and 6,219,246, listed below. U.S. Pat. No. 6,181,556, in particular, discloses an apparatus for electronic device heat dissipation consisting of a combination of heat sinks provided with fins, arranged so as to encircle the component to be cooled and thermally connected together by means of thermal bridge elements, these heat sinks being submitted to an airflow generated by several fans.
Utilization of heat pipes is also known for maintaining electronic components at suitable working temperatures. A heat pipe typically comprises a span of pipe closed at its ends and partially filled with heat-carrying fluid, the pipe comprising at least one evaporation region located close to or in contact with the heat source and at least one condensation region, for example, exposed to air which circulates the product in question. U.S. Pat. No. 6,226,178 listed below describes one embodiment of such a heat pipe. One drawback of a heat pipe system such as described in U.S. Pat. No. 6,226,178 is that the evaporation region is in contact with the single component or with an auxiliary board on which the component is assembled, but it does not effectively cool the entire printed circuit board on which a plurality of heat generating components are assembled.
One technology which provides a relatively high degree of heat dissipation for components of a circuit is referred to as the printed circuit board with insulated metal substrate technology, such technology commonly referred to in the art simply as IMS. A typical printed circuit board with insulated metal substrate comprises at least one metal support substrate, generally of an aluminum alloy, on which the electro-conducting circuit tracks, generally copper, are adhered by means of a layer of electrically insulating material which is the best possible heat conductor. With this arrangement, part of the heat generated by the components is dissipated through the metal substrate, which acts like a heat sink. However, in some power applications such as a control and distribution box in an automobile, the heat dissipation provided by the insulated metal substrate technology may be insufficient to ensure that a deterioration or malfunction of the circuit does not occur due to thermal fatigue.
Other approaches have been utilized to provide heat transference from heat-generating components utilized on or within circuitized substrates such as printed circuit boards. The following patents describe various ways, including those mentioned above, in which heat is withdrawn from electronic components used in conjunction with a printed circuit board. The list is not meant to be an exhaustive one of all possible structures, nor is it acknowledged that any of the listed documents are prior art to the present invention.
In U.S. Pat. No. 4,706,164, there is described a printed circuit card with a built-in heat exchanger. The card includes a mono- or multilayer circuit and heat drain integrated together, and having a system of channels for circulation of a heat-carrying fluid. The channels are formed on a plate and are enclosed by a closure plate.
In U.S. Pat. No. 5,875,097, a heat sink is mounted on a back plane of an auxiliary circuit board and the auxiliary circuit board is mounted on a main circuit board. In one embodiment, the heat sink is planer and extends between parallel auxiliary and main circuit boards. A finned heat dissipating member is at one side of the planar portion. Mounting pins in channels mount the heat sink to the main circuit board. The second embodiment is unshaped and stands on edge to mount the auxiliary circuit board perpendicular to the main circuit boards. Mounting pins in channels of the second embodiment likewise mount the heat sink to the main circuit board.
In U.S. Pat. No. 5,929,518, there is described a circuit board and method of transporting a conduction medium through the board for operating a semiconductor device supported on a surface of the board. A medium transport cavity is formed in the circuit board such that a portion runs parallel to the surface, and therefore has a length greater than the thickness of the board. The conduction medium can include air, a coolant fluid, or an optical fiber, which can transport energy such as pressure/vacuum, sound, heat or light from an external source to the semiconductor device.
In U.S. Pat. No. 5,998,240, there is described the cooling of densely packaged semiconductor devices in which micro channels extract heat by forced convection and fluid coolant is used as close as possible to the heat source. The micro channels maximize heat sink surface area and provide heat transfer, thereby allegedly allowing a higher power density of semiconductor devices without increasing junction temperature or decreasing reliability. In one embodiment, a plurality of micro channels is formed directly in the substrate portion of a silicon or silicon carbide chip or die mounted on a ground plane element of a circuit board where a liquid coolant is fed to and from the micro channels through the ground plane. The micro channels comprise a plurality of closed-ended slots or grooves of generally rectangular cross section. Fabrication methods include deposition and etching, lift-off processing, micromachining and laser cutting techniques.
In U.S. Pat. No. 6,032,355, there is described a method of manufacturing a thermal conductive structure on a printed circuit board which includes the steps of forming a thermal conductive layer having an embossed pattern on its surface, and then forming an adhesive glue layer over the thermal conductive layer. Next, a surface metallic layer is attached to the thermal conductive layer and the glue layer, wherein a portion of the surface metallic layer corresponding to the embossed portion of the heat spreader is in direct contact or almost direct contact with the thermal conductive layer. Furthermore, an additional external heat sink can be attached to thermal conductive structure for increasing the efficiency of heat dissipation.
In U.S. Pat. No. 6,129,145, there is described a heat dissipater which includes a heat-conductive substrate, a lid and a heat-conductive cover layer, and a coolant groove for passing a coolant there through. The groove is formed on a surface of the substrate. The lid is positioned on the coolant groove to seal the same, and the cover layer covers the surface of the substrate and the lid. The lid may be received in a lid groove to be flush with the surface. The substrate, lid and cover layer are all made of diamond, and are joined together with substantially no other material there between. Thus, high heat conductivity is achieved. The heat dissipater can be a heat dissipating substrate for an electronic component, or an optical transmission window.
In U.S. Pat. No. 6,181,556, there is described a thermally-coupled heat-dissipation apparatus which surrounds a solid state electronic device mounted on an adapter circuit board or a thermal transfer column extending upwardly from a main board socket device. The apparatus includes at least two heat sinks positioned in a spaced parallel relationship on front and back sides of the adapter circuit board or thermal transfer column. A bridge member thermally couples the two heat sinks. A plurality of fans is mounted along outer surfaces of the heat sinks. In one embodiment, the bridge member is an additional heat sink, but could be a metal plate.
In U.S. Pat. No. 6,190,941, there is described an electronic component which is mounted on a substrate such as a circuit board by means of a soldering process such as reflow soldering. The circuit board has a thermal via hole there through to provide a heat dissipation path from the top surface to the bottom surface of the circuit board, for dissipating heat from the electronic component. To prevent molten solder from penetrating through the via hole during the soldering process, the via hole is sealed prior to the soldering process. The via hole is sealed from the bottom surface of the substrate by carrying out a screen printing process including at least two printing passes to print a sealing material into the open hole of the thermal via.
In U.S. Pat. No. 6,201,300, there is described a printed circuit board thermal conductive structure which comprises a thermal spreader layer having an embossed pattern formed on its surface, an adhesive glue layer formed over the thermal spreader, and a surface metallic layer attached to the thermal spreader and the glue layer. A portion of the surface metallic layer is in direct contact or almost direct contact with the thermal spreader. Furthermore, an additional external heat sink can be attached to thermal conductive structure to increase the efficiency of heat dissipation.
In U.S. Pat. No. 6,201,701, there is described a substrate having a heat sink base and a multilayer circuit board bonded thereto, the multilayer circuit board having the capability to interconnect both power and control semiconductor elements and efficiently dissipate heat there from. A thermally conductive and electrically insulating bonding material having excellent thermal properties bonds the multilayer circuit board to the base. One or more thick electrically conductive foil patterns are formed intermediate the top surface of the circuit board and the heat sink base, the thick foil pattern(s) adapted to interconnect high power semiconductor elements. Also included intermediate the top surface of the circuit board and the heat sink base are one or more interconnect patterns. The substrate allegedly provides a compact density of control and power elements in one small module, while exhibiting good thermal properties so that heat generated by power semiconductors connected to the substrate can be dissipated.
In U.S. Pat. No. 6,212,071, there is described the use of several tracks of a heat conducting material embedded on a conventional printed circuit board and thermally connected on one end to those areas or components of the board where heat is generated and, on the other end, on the edge of the board to several external dissipaters, such as a metal casing encircling the board. These heat conducting tracks can be formed in a manner that is analogous to the electro-conducting tracks on a multi-layer board, in which case these would be of copper. These may also be formed by drilling several holes from the edge of the board and filling the holes with a melted metallic material, such as welding material. This patent also suggests that conventional heat pipes may be used.
In U.S. Pat. No. 6,219,246, there is described a heat sink apparatus for drawing heat from one or more devices and a method of attaching such a heat sink to one or more devices. The heat sink includes a mounting surface, which draws heat into the heat sink where it is dissipated by fins. The heat sink can be mounted next to the device to be cooled with minimum insertion force since the weight of the heat sink is borne by the printed circuit board upon which the electronic device is installed. A rotatable cam is turned by the user, which engages a pivot arm. The pivot arm rotates a number of spring clips against the device thereby holding it in place. Once in a fully closed position, the cam locks into place to prevent the pivot arm and spring clips from rotating back to an open position. The spring clips affix the heat sink and maintains contact between the mounting surface and the device being cooled. The individually articulated spring clips allow the heat sink to be mounted over multiple devices of various dimensions and locations along the heat sink.
In U.S. Pat. No. 6,226,178, there is described a computer including a microprocessor, an input coupled to provide input to the microprocessor, a mass storage coupled to the microprocessor and a memory coupled to the microprocessor to provide storage to facilitate execution of computer programs by the microprocessor. A heat pipe including spaced apart condenser regions and an evaporator region disposed between the condenser regions is provided. The evaporator region is positioned adjacent the microprocessor. A first heat dissipating device is attached to the heat pipe adjacent a first one of the condenser regions. A second heat dissipating device is attached to the heat pipe adjacent a second one of the condenser regions. The second heat dissipating device is of a different type than the first heat dissipating device.
In U.S. Pat. No. 6,414,847, there is described a integral dielectric heat spreader for transferring heat from semiconductor devices. A semiconductor device is mounted to a thermally conductive electrically insulating substrate, which forms the integral dielectric heat spreader. The heat spreader is then mounted to a package or a lower cost substrate such as a printed circuit board. The integral dielectric heat spreader may also support integral transmission lines, resistors, capacitors, or other bulk components. Performance of the heat spreader is enhanced through the use of thermal vias on a printed circuit board.
In U.S. Pat. No. 6,879,486, there is described a circuit board assembly which includes a cover or other member disposed adjacent the substrate and, for example, spaced there from so as to define a plenum. Self-aligning heat sinks (or other heat dissipative elements) are spring-mounted (or otherwise resiliently mounted) to the cover and, thereby, placed in thermal contact with one or more of the circuit components. Flow-diverting elements are provided, e.g., so that the overall impedance of the board substantially matches that of one or more of the other circuit boards in a common chassis. The circuit board cover can be adapted to provide thermal and/or electromagnetic emission control, as well as shock and vibration. A connector arrangement provides electrical, mechanical and/or other operational coupling between the circuit board and a chassis regardless of whether the board is disposed in a slot on a first (upper) side of a source of cooling air for the chassis or on a second (lower) side of this source. A circuit board can have one or two portions, each with an air flow inlet edge through which cooling air flow is received and an air flow outlet edge through which the air flow exists. A chassis for mounting of such circuit boards may have a center air inlet. It may also have a circuit-board insertion slot with a first air flow aperture disposed adjacent a first edge of an inserted circuit board and a second aperture disposed adjacent a second edge of the board. These apertures can be sized so that the impedance to air flow to a circuit board inserted in the slot substantially matches that to one or more other boards in the chassis. Such slots can form part of a card cage that is vacuum or dip brazed, or manufactured by an alternate process yielding a cage of desired structural stiffness and air-flow/interference sealing. The circuit boards and chassis can include an air and/or electromagnetic interference (EMI) seal which forms as the circuit board is inserted into the chassis slot.
In U.S. Pat. No. 7,119,284, there is described a printed circuit board having an insulated metal substrate with integrated cooling. The system includes a metal substrate, at least one electrically insulating layer adhered to the metal substrate and several electro-conducting tracks capable of interconnecting electronic power components, adhered to the electrically insulating layer. The metal substrate incorporates several heat transporting channels comprising several conduits for a heat-carrying fluid, conduits which extend to the outside of the metal substrate up to a heat transfer area to an outside medium.
In U.S. Pat. No. 7,176,382, there is described a multi-layer circuit board with heat pipes and a method for forming such a multi-layer circuit board. The method includes forming channels in a first and second pre-circuit assembly and attaching the first pre-circuit assembly to the second pre-circuit assembly such that the channels cooperatively form a heat pipe.
In U.S. Pat. No. 7,227,257, there is described an apparatus used for the cooling of active electronic devices utilizing micro channels or micro trenches, and a technique for fabricating the same. The micro channels run within a substrate and a stop layer, e.g., of diamond, is used in contact with (and even bound) the micro channels. Fluid passes through the micro channels and enters and exits the package.
As will be defined herein, the present invention represents a new and unique means for providing removal of heat from electronic components used in conjunction with a circuitized substrate. The thermal conductor used for this purpose may be formed as an integral part of the substrate and thermally coupled to one or more components. It may also be used as a common conductor if plating is used as part of the process of making the substrate. The process and resulting product as defined herein is of relatively low cost because it utilizes many processes known in the PCB art. It is even possible to use the invention in substrates having two opposing sides, each with one or more components mounted thereon.
It is believed that such a method and circuitized substrate would constitute significant advancements in the art.